We propose to develop a thermoacoustic-imaging device capable of making axial images of the cardiac anatomy in small animals such as mice and rats using near infrared light. The device we envision will allow researchers to assess cardiac dynamics and to study transgenic models of cardiovascular disease, such as cardiac hypertrophy and atherosclerosis. Ultimately we plan to develop a device capable of forming high-resolution images at a frame rate of 100 - 200 Hz, sufficient to image the cardiac dynamics of mice and rats. One advantage offered by such a device is that cardiac dynamics can be studied without the need for injecting vascular-enhancing contrast material. In Phase I, we will design, construct and test a focused, circular transducer array capable of collecting sufficient thermoacoustic data to reconstruct a 1-mm-thick axial slice following a single, 10-ns pulse of near infrared laser light. Because the laser pulse is so short, we should encounter virtually no blur due to heart-wall motion when imaging live animals. We hope to achieve in-plane spatial resolution of < 200 microns. Due to limitations of our present digital acquisition system (DAS), we will only have the capability of forming images at 2 frames per second during Phase I. In Phase II we will demonstrate our ability to form dynamic image sequences of the heart in mice and rats. To this end we will redesign our DAS to allow us to acquire dynamic image sequences at rates as high as 200 Hz. We will also incorporate a custom-designed, tunable, near-infrared laser capable of pulsing at 200 Hz. In this phase we will develop multi-slice imaging protocols to form 3-D images of the beating heart.